The downtown “convadium” proposed by the Chargers in Measure C could pump $2.1 billion into the San Diego economy and create 15,000 jobs during construction — as well as support $400 million of activity and 6,400 permanent jobs each year thereafter, concludes an independent economic analysis funded by the team to be released today.

For perspective, the project would represent merely a tiny sliver of San Diego County’s overall economy, which totaled nearly $221 billion in 2015 and employed 1.4 million people.

Still, the study’s result was surprising, on the upside. Put simply, the analysis suggests that Measure C is not your typical National Football League boondoggle.

Most taxpayer-funded sports stadiums deliver zero (or even negative) lasting economic improvement in a city, for the simple reason that family entertainment budgets are finite. If you spend cash to attend an NFL game, you’re not going to the movies or the zoo that day.

The Chargers $1.8 billion proposal is different. Along with building a stadium, it would add 385,000 leasable square feet to the city’s maxed-out convention center space, attracting hundreds of millions of new visitor dollars from outside the region.

I know what you’re thinking. When private developers beg for taxpayer funding, a rosy economic study is sure to follow.

Yet this one is more credible than most, in my view. The authors were Alan Gin, the Ph.D. economist at University of San Diego, and Murtaza Baxamusa, a Ph.D. planner who works for a union-backed housing nonprofit, along with USD research assistant Katelyn Allende.

These academics are not in the tank for developers. The Chargers hired them in June to conduct the economic study and promised them no interference or editing of their report.

Just as important, Gin and Baxamusa stuck to standard “multipliers” developed by the Bureau of Economic Analysis, an arm of the U.S. Commerce Department that develops San Diego-specific methodology for estimating how dollars injected into the regional economy produce jobs.

“We did not want to put anything out there that can’t be independently checked by anybody else,” Baxamusa said. “We wanted it (the analysis) to be academically robust.”

Such rigor led to estimates that strike me as conservative, with a few caveats I’ll get into below. Gin said the analysis focused on money that would be new to San Diego, or would leave town if the Chargers leave. By nature, such estimates are inexact.

For example, Measure C would raise the city’s tax on hotel stays to 16.5 percent from 12.5 percent, raising enough annual revenue to support an estimated $1.15 billion in construction spending (with some left over for operations and maintenance). Because hotels are generally occupied by visitors, the study counted all this money as “direct” spending from outside the region.

Similarly, the NFL would contribute $300 million, while the Chargers would spend $350 million from the team, fans and sponsors. I can make the case that the private side of the ledger may include about $100 million in “stadium builder” advance payments from fans inside the county, but the quibble doesn’t affect the result much.

As for the project itself, roughly $200 million would go for land acquisition, which has no economic multiplier. Of the remaining $1.6 billion, about $250 million may go for soft costs like professional fees, financing and insurance.

That leaves $1.35 billion for construction, which the BEA calculator says can create 12,467 jobs over the project’s roughly three years of construction for everything from steelworkers or carpenters onsite to extra clerks and truck drivers at local suppliers. Such activity would also create 2,544 indirect jobs, reckon the study’s authors, with the total workforce of about 15,000 earning $928 million in labor income.

Critics of Measure C argue, persuasively in my view, that $200 million isn’t nearly enough to buy all the land and move a regional bus yard from the project’s East Village site. If true, the required public spending would be higher, although neighboring cities may contribute.

Building a new bus complex would generate economic activity, too. Yet Gin and Baxamusa left such activity out of their study, because the number is unknowable.

They also didn’t include the Chargers’ payroll, which included an estimated $155 million of player salaries in 2014, because there’s no credible way to assess how much of that cash is spent inside San Diego County.

Both decisions tend to understate the economic activity from the project.

This brings us to the less clear-cut matter of the project’s ongoing economic contribution, which is more important than the one-time construction splurge.

Gin and Baxamusa decided to include the Chargers’ annual operations, which pump an estimated $104 million into the local economy and support 1,628 jobs. This assumes the team, which has permission from the NFL to share a new venue with the Rams in Inglewood, will leave town if San Diego doesn’t help them build the project envisioned in Measure C.

The rest of the project’s permanent economic uplift would owe to the operation of the convention center, contributing an estimated 4,772 jobs and nearly $300 million in total activity.

The convention center piece of the study seems likely to draw criticism. That’s because it leans heavily on forecasts prepared by tourism-industry consultants hired by the Chargers. Competing studies, funded by the hotel industry and the city, have forecast that a separate, stadium-attached convention center would be economically inferior to expanding the bayside center.

At the same time, such criticism employs an old trap of logic: Given any real proposal, it’s easy to argue for a superior project that doesn’t exist.

The Chargers have argued that no purely private development can attract enough capital to move the bus yard and replace 1,100 parking spots leased by the Padres near Petco Park.

A better argument involves opportunity cost. If the Chargers build a stadium downtown, the city would be free to demolish Qualcomm Stadium in Mission Valley and sell or donate the site for a major expansion of San Diego State University or UC San Diego.

When it comes to economic multipliers, nothing comes close to higher education. A reputable study in 2007 found that adding 10,000 students to SDSU would boost the regional economy by more than $2 billion a year, a figure that’s surely higher now.

However, the numbers may do more to obscure the basic decision facing voters with Measure C.

Cities build stuff all the time, from parks to libraries, that yield no direct economic benefit. Other government endeavors offer payoffs that are huge yet impossible to measure.

The economic benefit of fundamental public safety is close to infinite, as the Syrians have learned by destroying theirs. At the other extreme, adding one new police station may deliver no tangible improvement in law and order.

As I’ve argued in columns since early 2015, the best reason to support the Chargers is because you want a cool new stadium, period. In contrast, if you loath subsidies for billionaires, Measure C is a terrible idea.

Studies by experts of relatively small “impacts” are interesting, but may be beside the point.